The present invention relates to a digital-to-analog (hereinafter referred to as D/A) converting apparatus which permits conversion of a digital signal of many bits into an analog signal with high accuracy.
A known D/A converter is of the type in which a plurality of current sources are provided for generating binary weighted currents and, in accordance with an input digital signal, the current sources of the binary-weighted currents are controlled by the corresponding binary bits of the input digital signal to permit or cut off the supply of the binary-weighted currents and then the output currents are added together, thereby to obtain an analog converted output. In such a D/A converting apparatus, when the number of bits of the digital signal to be converted is large, the currents of the current sources corresponding to the least significant bit and the most significant bit of the digital signal markedly differ, which makes it difficult to construct a current source for the most significant bit which yields a current with an error equal to or smaller than that of the current of the current source corresponding to the least significant bit.
To avoid this, it is a general practice in the prior art to convert a plurality of high-order bits of the input digital signal, including the most significant bit, to prepare a plurality of signals for a respective plurality of constant-current sources corresponding to the high-order bits, each of which generates a current corresponding to the least significant one of the high-order bits, and to simultaneously output a number of currents from the high-order bit constant-current sources according to the converted plurality of signals. With this method, it is possible to achieve high-precision D/A conversion and to decrease spike noise, i.e. what is called a glitch, which is generated during switching of the input digital signal.
With this method, however, the current value of the high-order bit current source is very much larger than the current value of the current source corresponding to the least significant bit. Consequently, high precision is needed and it is difficult to hold the current ratio of each current source to the others constant, resulting in degraded linearity of the conversion characteristic. Adjustment for holding the current ratio as predetermined is troublesome and time-consuming and, in addition, this adjustment is often required for each ambient temperature change or secular change.